Economizer bypass for increased furnace wall protection

ABSTRACT

An arrangement for protecting, at low loads, the furnace walls of a once-through steam generator system, having a recirculation loop, by increasing the mass flow of the working fluid flowing through the furnace wall tubes. A bypass for incoming feedwater is provided around the economizer allowing a lower temperature fluid to be delivered to and mixed with the fluid from the recirculation loop lowering the temperature of the fluid mixture. This results in a decrease in the specific volume of the mixed fluids; and since the recirculating pumps in the recirculation loop supply almost the same volumetric flow under all loads to the furnace wall tubes, the lower specific volume of the mixed fluid delivered to the pumps will yield a greater mass fluid flow to the furnace walls. The increased mass flow in turn results in improved heat transfer characteristics within the furnace walls thus reducing the likelihood of furnace wall damage due to overheating.

United States Patent [191 Clayton, Jr. et a1.

ECONOMIZER BYPASS FOR INCREASED FURNACE WALL PROTECTION Inventors:William Harold Clayton, Jr.,

Windsor, Conn; Wolfram Gerhard Schuetzenduebel, Santa Fe, Calif.

Assignee: Combustion Engineering Inc.,

Windsor, Conn.

Filed: June 29, 1973 Appl. No.: 375,219

US. Cl 122/406 S, 122/451 S Int. Cl. F22b 29/12 Field of Search 122/406S, 406 ST, 412

122/421, 448 S, 451 S, 479 S References Cited 1 UNITED STATES PATENTSPrimary Examiner--Kcnncth W. Sprague Attorney, Agent, or FirmRobert L.Olson 15 7] ABSTRACT An arrangement for protecting, at low loads, thefurnace walls of a once-through steam generator system, having arecirculation loop, by increasing the mass flow of the working fluidflowing through the furnace wall tubes. A bypass for incoming feedwateris provided around the economizer allowing a lower temperature fluid tobe delivered to and mixed with the fluid from the recirculation looplowering the temperature of the fluid mixture. This results in adecrease in the specific volume of the mixed fluids; and since therecirculating pumps in the recirculation loop supply almost the samevolumetric flow under all loads to the furnace wall tubes, the lowerspecific volume of the mixed fluid delivered to the pumps will yield agreater mass fluid flow to the furnace walls. The increased mass flow inturn results in improved heat transfer characteristics within thefurnace walls thus reducing the likelihood of furnace wall damage due toover heating.

6 Claims, 3 Drawing Figures /-SUPEl2HEATER 1 TO CONDENSER fi l FURNACEWALLS TO CONDENSER FURNACE WALL MASS FLOW /0) PATENTEUJUNZS'QY $818,872

SHEET 2 or 2 20 40 so so I00 ECONOMIZEIZ BYPASS LINE FLOW /0) F IG F 2.

O. 5 Ho F. 3 3 LL.

D Q 700. i

o 2o 4 60 8o I00 ECONOMIZER BYPASS LINE FLOW FIG-3 BACKGROUND ANDSUMMARY OF THE INVENTION This invention relates to once-through steamgenerator systems with recirculation through the furnace wall tubes, andmore particularly to protection of the furnace wall tubes by increasingthe mass flow therethrough.

In the start-up and low load operation of a oncethrough steam generator,it is necessary that fluid flow be established through the fumace walltubes in order to protect the tubes from damage due to the heatgenerated by fuel firing. One present method of establishing this flowis recirculating a portion of the working fluid through the furnace walltubes, in the manner shown and described in US. PatsNo. 3,135,252 to W.W. Schroedter. Protection according to this method is not entirelyadequate for the desired furnace wall protection. It is, therefore, thepurpose of our invention to provide improved furnace wall protectionwith minimum change to the existing arrangement.

During start-up and low load operation the fluid passed through thefurnace walls of a once-through steam generator with recirculationconsists of a mixture of heated feedwater from the economizer andrecirculated working fluid from the furnace wall outlet. Little controlof the temperature of this mixed fluid is possible due to the fact thatthe furnace wall outlet temperature is maintained close to designconditions and the economizer outlet temperature is a function ofcombustion gas temperature. Since, at start-up and low load, only asmall degree of cooling is obtained by the above-noted mixing (due tohigh economizer temperatures and low percent through-flow),recirculation does not provide a sufficient heat transfer relationshipbetween the circulating mixed fluid and the furnace wall tubes for theadequate protection thereof.

Our inventionprovides an arrangement for improving furnace wallprotection including a bypass for at least a portion of the incomingfeedwater around the economizer whereby the temperature of the mixtureof the furnace wall outlet fluid and the feedwater from the economizeroutlet plus the bypassed feedwater, as delivered to a recirculationpump, is reduced. This fluid temperature reduction permits the amount offluid mass flow in the furnace wall tubes (and thus the heat transfercharacteristics thereof) to be increased. Restated, the reduction intemperature of the fluid mixture delivered to the recirculation pumpsreduces the specific volume thereof. The volumetric flow of the pumps issubstantially the same under any load condition, so that a lowerspecific volume results in a higher mass flow to be delivered to thefurnace walls. Since higher mass flow creates better heat transfercharacteristics, the furnace wall tubes are more able to stand the heatof fuel tiring and are thus more adequately protected.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammaticrepresentation of the flow arrangement of this invention.

FIG. 2 is a graphical representation of the increase in furnace wallmass flow with the increase in economizer bypass line flow.

FIG. 3 is a graphical representation showing the decrease in mixed flowtemperature with increase in economizer bypass line flow.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, duringgeneral operation of the once-through steam generator systemincorporating our novel arrangement, feedwater is supplied through aconduit 12 to a feedwater supply valve 14. A flow meter 16 provides asignal for an operator 18 which controls the flow through the feedwatersupply valve 14. This water is passed serially through an economizer 20to a mixing vessel 20, and thence through recirculation pump 24 to thefurnace wall tubes 26 where it is heated by combustion within thefurnace to form steam. The steam leaving the fumace tubes passes throughconduit 28 to a superheater 30 from where it is taken through conduit 32to do useful work such as to operate a turbine (not shown).

A feedwater valve bypass portion 34 having a bypass valve 36 is providedfor controlling feedwater flow at very low loads. In parallel with thefeedwater supply valve 14 and feedwater valve bypass portion 34 is theeconomizer bypass portion 38, of our novel arrangement. Economizerbypass portion 38 has an economizer bypass valve 40 which enables aportion of the feedwater to be bypassed-around the economizer 20 andflow directly into the mixing vessel 22 at low loads as describedhereinbelow. It is pointed out that economizer bypass valve 40 mustbeparalllel to feedwater bypass valve 36 in order to equalize thepressure drop across the economizer 20 and bypass portion 38 to achievethe bypass flow.

At the beginning of start-up of the once-through steam generator system,cold water is recirculated through conduit 42 from the furnace wall tubeoutlet 44 to the mixing vessel 22 where it is mixed with incomingfeedwater when a nominal through-flow of cold water is established.Nominal through-flow (usually about 5 percent of designed full loadthrough-flow) is used to provided improved pressure control and uniformheating of various tubing and headers outside of the recirculating flowpath. As the furnace is tired (and load applied), the temperature of thefluid in the furnace wall tubes 26 and in the economizer 20 increases.The temperature of the substantially recirculated flow will thenincrease to a relatively high temperature (approximately 760 to 800F).The mixed flow (consisting of fluid from the recirculating conduit andfrom the economizer) entering the furnace walls 26 from the mixingvessel 22 will then be at a high temperature and a low density, thedensity varying inversely with the temperature. For example, at 3,500psi and 200F, the density is 61.5 lb/ft? while at 800F it is 7 lb/ft.

While the volumetric flow rate during this start-up and low loadoperation is substantial, the mass flow rate is low due to the lowdensity of the mixed fluid. The metal temperature of the furnace side ofthe furnace wall tubes is dependent not only on the rate of heattransfer from the furnace gases to the tubes but also the thermalresistance of the tubes and the inside film between each tube and thefluid therein. Since this inside film is a function of mass flow raterather than velocity or volumetric flow rate, poor heat transfer betweenthe tubes andthe fluid is obtaineddu'ring this period of operation.Therefore, while the furnace is operated at low loads and relatively lowheat adsorption rates (as compared to full load), high local heatingrates still may exist in the general area of the burners which can causedamage to the tubes located in this area.

As the amount of through-flow is increased the recirculation isdecreased, a greater quantity of incoming feedwater is mixed with alesser quantity of the relatively hot recirculated water. The mixedwater temperature, thus decreases and density increases as the load onthe steam generator is increased. Therefore, safe operating conditionsfor the furnace wall tubes are easily achieved at higher loads wherethere is substantial through-flow and adequate heat transfercharacteristics exist.

A critical condition exists at the low loads up to 30 percent of fullload when there exists substantial firing in the furnace and a highquantity of recirculation which leads to high temperatures and low massflow in the furnace wall tubes. According to our arrangement, in orderto increase the mass flow through the furnace wall tubes 26 and thusimprove the heat transfer characteristics therein, a portion of thefeedwater is bypassed through economizer bypass portion 38 around theeconomizer 20 and directly into the mixing vessel 22. The temperature ofthe bypassed feedwater is approximately 220F as compared to feedwaterpassed through the economizer which is at a temperature of approximately500F. This lower temperature bypassed feedwater reduces the temperatureof the mixed fluid which increases its density and mass flow to be delivered to the furnace wall tubes 26.

A temperature sensing device 46 is located between the mixing vessel 22and the recirculation pump 24 to sense the temperature of the mixedfluid delivered to the recirculation pump 24. The sensed temperature iscompared in a controller 48 to a Set Point temperature (desired optimumworking fluid temperature) to control an operator 50 according to thedifference, or error, signal generated for regulating the amount offeedwater bypassed around the economizer 20 at the above-mentionedloads. As the temperature of the recirculated flow rises, raising thesensed mixed fluid temperature above the Set Point temperature, theamount of fluid bypassing the economizer is increased. This increasedtemperature increased bypass association is continued up to about 30percent load where substantial through-flow will provide sufficient tubewall protection. Therefore, above this point (about 30 percent load)economizer bypass is no longer necessary for tube protection and isdiscontinued.

Referring to the graph of FIG. 2, it can be seen that the mass flowthrough the furnace wall tubes can be increased up to a maximum of about123 percent of normal mass flow by the bypassing of the feedwater aroundthe economizer. As noted in the graph of FIG. 3, the temperature ofmixed fluid can be lowered from 718 to 715F. While a three degree dropin temperature does not appear large, the specific volume changeassociated therewith is substantial as reflected in the increased massflow shown in the graph of FIG. 2.

Thus it can be seen that by bypassing a portion of the feedwater aroundthe economizer, the temperature of the mixed fluid circulating in thefurnace walls can be reduced. The reduction in temperature results inincrease in mass flow which has the effect of promoting better heattransfer characteristics between the tube walls and fluid therein forimproved furnace wall protection during the period of low loads whenother protective methods, valid at higher loads, have been foundinsufficient.

It will be understood that various changes in the details, andarrangements of parts which have been herein described and illustratedin order to explain the nature of the invention, may be made by thoseskilled in the art within the principle and scope of the inventionexpressed in the appended claims.

What is claimed is:

1. In a once-through steam generator system comprising a furnace withfuel being burned therein establishing the flow of combustion gasestherefrom, furnace wall tubes lining the walls of said furnace forradiant heat absorption from the burning fuel within said fur nace, heatexchange means in the flow of combustion gases, a first conduit forconveying incoming working fluid into said heat exchange means, acontrol valve in said first conduit for controlling the flow of incomingworking fluid, a second conduit for conveying said working fluid to saidfurnace wall tubes from within said heat exchange means, a third conduitfor conveying the working fluid from said furnace wall tubes afterpassage therethrough, a fourth conduit providing a recirculation pathfor the working medium from said third conduit to said second conduitand pump means for recirculating at least a portion of the working fluidthrough said fourth conduit; an arrangement for protecting said furnacewall tubes comprising:

bypass means for selectively bypassing at least a portion of theincoming working fluid around said heat exchange means so that saidworking fluid enters said furnace walls without being heated in saidheat exchange means, said bypass means comprising a fifth conduitconnected between said first conduit and said second conduit, aselectively operable valve means in said fifth conduit for regulatingthe flow therein, said pump means being located in said second conduitdownstream of the connection of said fourth and fifth conduits with saidsecond conduit.

2. The arrangement of claim 1 wherein said selectively operable valvemeans is located so as to be in parallel with said control valve.

3. The arrangement of claim 1 wherein automatic control means areprovided to control said selectively operable valve means of said bypassmeans, said automatic control means responsive to a condition of theworking fluid as it enters said furnace walls.

4. The arrangement of claim 3 wherein said automatic control meanscomprise a temperature sensing means sensing the temperature of theworking fluid entering said furnace wall tubes, said means locateddownstream of the junction of said fourth conduit with said secondconduit and said fifth conduit with said second conduit and upstream ofsaid pump means, controller means receiving a signal from saidtemperature sensing means indicating temperature of the working fluid,said controller means also receiving a Set Point signal indicating thedesired working fluid operating temperature, said controller meansoperative in response to the difference between said received signal tocontrol the regulative position of said selectively operable valve meansover at least a portion of the steam generator operating load range.

5. A method of operating a once-through steam generator system having afurnace, furnace wall tubes lin- 6 ing the walls of the furnace forradiant heat absorption, omizer is substantial at low loads anddiscontinued and an economizer, the steps comprising: at high loads.

establishing a through-flow of incoming working fluid 6. The method ofclaim 5 wherein the step of regulatthrough said economizer and saidfurnace wall ing said bypassed portion comprises the steps: sensingtubes, recirculating a portion of the flow from the 5 the temperature ofthe mixed working fluid flow as it outlet of the furnace wall tubes tothe entrance of enters the furnace wall tubes, comparing the sensed thefurnace wall tubes, mixing the recirculated flow temperature to anoptimum desired temperature, con and the through-flow upstream of thefurnace walls trolling the bypassed flow portion during load operaanddownstream of the economizer, firing fuel in tion up to 30 percent offull load according to the difsaid furnace thereby increasing thetemperature of 0 ference between the sensed temperature and the dethemixed working fluid passing through said fursired temperature so thatthe flow around said econonace wall tubes, bypassing at least a portionof the mizer may be increased, and discontinuing said byincoming workingfluid through-flow around said passed flow portion during operating loadconditions economizer, regulating said bypassed portion so above 30percent of full load. that the amount of flow bypassed around said econ-

1. In a once-through steam generator system comprising a furnace withfuel being burned therein establishing the flow of combustion gasestherefrom, furnace wall tubes lining the walls of said furnace forradiant heat absorption from the burning fuel within said furnace, heatexchange means in the flow of combustion gases, a first conduit forconveying incoming working fluid into said heat exchange means, acontrol valve in said first conduit for controlling the flow of incomingworking fluid, a second conduit for conveying said working fluid to saidfurnace wall tubes from within said heat exchange means, a third conduitfor conveying the working fluid from said furnace wall tubes afterpassage therethrough, a fourth conduit providing a recirculation pathfor the working medium from said third conduit to said second conduitand pump means for recirculating at least a portion of the working fluidthrough said fourth conduit; an arrangement for protecting said furnacewall tubes comprising: bypass means for selectively bypassing at least aportion of the incoming working fluid around said heat exchange means sothat said working fluid enters said furnace walls without being heatedin said heat exchange means, said bypass means comprising a fifthconduit connected between said first conduit and said second conduit, aselectively operable valve means in said fifth conduit for regulatingthe flow therein, said pump means being located in said second conduitdownstream of the connection of said fourth and fifth conduits with saidsecond conduit.
 2. The arrangement of claim 1 wherein said selectivelyoperable valve means is located so as to be in parallel with saidcontrol valve.
 3. The arrangement of claim 1 wherein automatic controlmeans are provided to control said selectively operable valve means ofsaid bypass means, said automatic control means responsive to acondition of the working fluid as it enters said furnace walls.
 4. Thearrangement of claim 3 wherein said automatic control means comprise atemperature sensing means sensing the temperature of the working fluidentering said furnace wall tubes, said means located downstream of thejunction of said fourth conduit with said second conduit and said fifthconduit with said second conduit and upstream of said pump means,controller means receiving a signal from said temperature sensing meansindicating temperature of the working fluid, said controller means alsoreceiving a Set Point signal indicating the desired working fluidoperating temperature, said controller means operative in response tothe difference between said received signal to control the regulativeposition of said selectively operable valve means over at least aportion of the steam generator operating load range.
 5. A method ofoperating a once-through steam generator system having a furnace,furnace wall tubes lining the walls of the furnace for radiant heatabsorption, and an economizer, the steps comprising: establishing athrough-flow of incoming working fluid through said economizer and saidfurnace wall tubes, recirculating a portion of the flow from the outletof the furnace wall tubes to the entrance of the furnace wall tubes,mixing the recirculated flow and the through-flow upstream of thefurnace walls and downstream of the economizer, firing fuel in saidfurnace thereby increasing the temperature of the mixed working fluidpassing through said furnace wall tubes, bypassing at least a portion ofthe incoming working fluid through-flow around said economizer,regulating said bypassed portion so that the amount of flow bypassedaround said economizer is substantial at low loads and discontinued athigh loads.
 6. The method of claim 5 wherein the step of regulating saidbypassed portion comprises the steps: sensing the temperature of themixed working fluid flow as it enters the furnace walL tubes, comparingthe sensed temperature to an optimum desired temperature, controllingthe bypassed flow portion during load operation up to 30 percent of fullload according to the difference between the sensed temperature and thedesired temperature so that the flow around said economizer may beincreased, and discontinuing said bypassed flow portion during operatingload conditions above 30 percent of full load.